1. Field of the Invention
The present invention relates to a catalytic system and the process using this system for dehydrogenating ethylbenzene to styrene.
2. Background of the Invention
Styrene is an important intermediate for the production of plastic materials and rubber.
It is mainly used in the production of polystyrenes (GPPS crystals, shock-resistant HIPS and expandable EPS), acrylonitrile-styrene-butadiene (ABS) and styrene-acrylonitrile (SAN) copolymers of styrene-butadiene rubbers (SBR).
At present styrene is mainly produced by means of two processes:
by the dehydrogenation of ethylbenzene (EB) (which covers about 90% of the world capacity (App. Cat. 133, 1995, 219)); PA1 as a coproduct in the epoxidation of propylene with ethylbenzene hydroperoxide with catalysts based on molibden complexes. PA1 the oxidative dehydrogenation of ethylbenzene; PA1 the dehydrogenation of ethylbenzene followed by the oxidation of hydrogen. PA1 the use of several adiabatic reactors in series, with intermediate heating steps, in which the temperature is between 540 and 630.degree. C. with contact times more or less of tenths of a second; PA1 The use of radial flow reactors operating under vacuum in which the pressure is between 0.3 and 0.5 atm; PA1 the use of water vapor in co-feed with the charge to be dehydrogenated. PA1 it reduces the partial pressure of the products and therefore favourably shifts the thermodynamic equilibrium; PA1 by the reaction of water gas, it contributes to decoking the catalyst, as there is no burn-off of the catalyst with air; PA1 it supplies all the heat necessary for the dehydrogenation of EB. PA1 use of huge quantities of superheated vapor (H.sub.2 O/EB=9.0-9.8 (molar) with a temperature of over 700.degree. C.: this necessitates the use of superheating ovens and therefore high investment costs; PA1 aging of the catalyst: this makes it necessary to replace it after about 18-36 months of operation; to do this it is necessary to stop the unit and interrupt production for the period required for its substitution; it is possible to prolong the life by increasing the ratio H.sub.2 O/EB, but this further compromises the energy balance; PA1 recuperation of energy not as yet optimized: the present technologies, in fact, only recuperate the sensitive heat of the vapor and not also the latent heat; PA1 carrying out the reaction under vacuum (average absolute pressure of 0.4 atm) and therefore in an extremely dilute phase in EB: the partial pressure of the EB is on an average equal to 0.04 atm. PA1 the chromium expressed as Cr.sub.2 O.sub.3, is in a quantity of between 6 and 30% by weight, preferably between 13 and 25%; PA1 the tin, expressed as SnO, is in a quantity of between 0.1 and 3.5% by weight, preferably between 0.2 and 2.8%; PA1 the alkaline metal, expressed as M.sub.2 O, is in a quantity of between 0.4 and 3% by weight, preferably between 0.5 and 2.5%; PA1 the silica is in a quantity of between 0.08 and 3% by weight, the complement to 100 being alumina. PA1 addition of the tin to the carrier before the dispersion of the precursors of chromium and potassium oxide; PA1 treatment of the solid containing chromium and potassium oxide by ionic exchange, impregnation, etc., with a solution containing a compound of tin; PA1 deposition of the tin via "vapor deposition" onto the carrier, before adding the precursors of chromium oxide and potassium oxide, using a volatile compound of the species to be deposited; PA1 deposition of the tin via "vapor deposition" onto the solid containing alumina, chromium oxide and potassium oxide, using a volatile compound of the species to be deposited. PA1 a) reacting in a reactor, operating at a temperature of between 450 and 700.degree. C., at a pressure of between 0.1 and 3 atm and with a GHSV space velocity of between 100 and 10000 h.sup.-1 (normal liters of hydrocarbon/h.times.liter of catalyst), the ethylbenzene with the catalytic system described above, preferably diluted with an inert product at a weight concentration of the catalytic system of between 5 and 50%. PA1 b) regenerating the catalytic system in a regenerator by burning the coke deposited during the reaction phase operating at a temperature of over 400.degree. C. PA1 at a temperature maintained, by acting on the flow-rate of the regenerated catalyst, at between 450 and 650.degree. C. depending on the reaction desired; PA1 at a pressure which is atmospheric or slightly higher; PA1 at a space velocity of between 100 and 1000 h.sup.-1 (Nliters of ethylbenzene and inert gas per hour and per liter of catalyst), more preferably between 150 and 200; PA1 with a residence time of the catalyst varying in the fluid bed zone from 5 to 30 minutes, more preferably from 10 to 15 minutes, in the desorption zone from 0.2 to 10 minutes. PA1 the heat is directly transferred to the reaction by the regenerated catalyst: there are no superheating ovens for the thermal exchange and the strong remixing of the fluid bed prevents the formation of high temperature points which would lower the selectivity; PA1 the fluid bed process makes it possible to recycle hydrogen; PA1 all the other operations are carried out in continuous and it is not necessary to modify the operating parameters during the whole life of the plant; PA1 the plant can operate with wide flexibility in terms of present productive capacity with respect to the project capacity; PA1 the reaction and regeneration take place in physically separated zones and there cannot be any mixing of hydrocarbon streams with streams containing oxygen; PA1 the process is carried out at atmospheric or a slightly higher pressure: there is therefore no possibility of external infiltrations of air into the reaction zone; PA1 no particular treatment is necessary for reducing the emissions of gaseous pollutants; PA1 the molar concentration inert products/ethylbenzene in the feed is much lower than in commercial technologies.
Two alternative ways of producing the monomer have recently been studied and in some cases developed on an industrial scale:
We shall now only consider the production of styrene by the dehydrogenation of ethylbenzene as this is the method followed by the process of the present invention.
The dehydrogenation reaction of ethylbenzene to styrene has various particular characteristics which should be taken into consideration for the technological design.
The first is that the reaction is controlled by the thermodynamic equilibrium and therefore the conversion per passage cannot be total.
The degree of dehydrogenation increases with the rise in temperature and reduction of the total pressure, the reaction taking place, at a constant pressure, with an increase in volume.
To obtain economically acceptable conversions, the thermodynamics makes it compulsory for the reaction to be carried out within the range of 540-630.degree. C. It is also necessary to operate in the presence of a suitable catalyst owing to the low rate at which the ethylbenzene dehydrogenates, also at these thermal levels.
Owing to the rather high operating temperatures, parasite reactions inevitably take place, these generally being characterized by a greater activation energy with respect to the dehydrogenation energy. The main product is therefore accompanied by by-products, mainly consisting of toluene, benzene, coke and light products. The function of the catalyst is to direct the reaction towards the desired product.
The last important aspect consists in the fact that the reaction is strongly endothermic, with a reaction heat equal to 28 Kcal/mole of styrene, corresponding to 270 Kcal/kg of styrene produced. The high heat required and high thermal levels at which it must be exchanged are the aspects which mainly influence the technological design. The technologies at present sold (Fina/Badger and Lummus/UOP Classic SM processes) satisfy the demands imposed by the thermodynamics by adopting a technological system which comprises:
Water is the main component being fed to the reactor. The typical molar concentration is 90%. Often however a concentration of more than 90% is adopted to lengthen the chemical life of the catalyst.
The vapor has several functions:
With present technologies, conversions of 60-65% are reached with selectivities to styrene of more than 90% by weight with an optimized catalyst mainly based on iron oxide promoted with alkalies.
In spite of the performances, the present technologies have disadvantages which are mainly due to the following aspects: